When printing is effected on a predetermined sheet by means of a printer, if the paper is displaced during the printing operation, the print may be blurred. In order to prevent the occurrence of this problem, it is general practice to provide a printer with a sheet securing mechanism which is designed to firmly retain a sheet.
FIGS. 8 and 9 show in combination one example of a conventional paper securing mechanism. In these FIGS., the reference numeral 1 refers to an elongated flap plate-shaped lower base which is fixedly provided on a printer. Flanges 1a are provided at two longitudinal ends, respectively, of the lower base 1, and pivot shafts 2 are respectively provided on the flanges 1a. The numeral 3 refers to a flat plate-shaped upper arm which is substantially equal in length to the lower base 1, the upper arm 3 being pivotally attached to the lower base 1. More specifically, flanges 3a are formed at two longitudinal ends, respectively, of the upper arm 3, and these flanges 3a are pivotally supported by the pivot shafts 2 of the lower base 1. One side edge portion 3b of the upper arm 3 has a series of projections 3c of equal width. An upper friction member 4 is rigidly secured to that surface of each of the projections 3c which faces the lower base 1. The other side edge portion 3d of the upper arm 3 is slightly bent so as to extend away from the lower base 1.
Springs 5 are respectively fitted on the pivot shafts 2 [see FIG. 9a]. The springs 5 bias the upper arm 3 so that the upper friction members 4 are pressed against the lower base 1.
A disc cam 6 is disposed above the side edge portion 3d of the upper arm [see FIGS. 9b and 9c]. Counterclockwise rotation (as viewed in FIG. 9) of the cam 6 causes it to engage with the side edge portion 3d of the upper arm 3, thus pushing down the side edge portion 3d. More specifically, the rotation of the cam 6 causes the upper arm 3 to pivot clockwise about the pivot shafts 2 against the biasing force of the springs 5 [see FIG. 9b]. Thus, the upper friction members 4 which are secured to the side edge portion 3b are separated from the lower base 1.
In the state wherein the upper friction members 4 are separated from the lower base 1, the leading edge of a sheet 7 is placed on a portion of the lower base 1 which is directly below the upper friction members 4 [see FIG. 9b]. As the cam 6 is further rotated, the upper arm 3 is pivoted counterclockwise by means of the biasing force of the springs 5, thus causing the upper friction members 4 to press the leading edge of the sheet 7 against the lower base 1 [see FIG. 9c]. As a result, the edge of the sheet of paper 7 is firmly clamped between the upper friction members 4, and the lower base 1 and rigidly secured by means of friction forces produced between the sheet 7 and the members 4 and the lower base 1.
In order to obtain large frictional force in such a conventional paper securing mechanism, it is necessary to increase the area of the surface used for clamping, that is, the total area of the lower surfaces of the upper friction members 4 and the total area of the portions of the lower base 1 which correspond to the lower surfaces. In other words, the overall size of the mechanism must be increased. In addition, in order to obtain large frictional forces simultaneously, it is necessary to make the springs 5 quite stiff. For this reason, each portion of the mechanism has heretofore been produced by using materials which are sufficiently strong to bear this relatively large spring biasing force. If a print head moving at high speed comes into contact with the mechanism, the head may be damaged.
Generally, the surface condition of sheet changes in accordance with variations in environmental conditions such as temperature and humidity. Accordingly, conventional sheet securing mechanism have a problem in that the frictional force may be lowered considerably due to a variation in the environment so that a sheet of paper cannot reliably be secured.